K40 Laser Machine Laser Power Control - Two output & One Input voltage

[mraoofi]

Folks,

I am not experienced and really need some help here; please pardon my ignorance and inaccurate questions/comments/assumptions below (and also the crudely drawn circuit).  Here are the details of my issue...

Problem Statement:
I have two DC voltage sources, where both can be from 0 to 5V, I want to compare the two and always select the lower input (and if they are equal then any source will do).


Some background details on this project…

This is for a cheap Chinese K40 Laser Machine.  I am replacing the stock controller board with a better control board allowing me to use it with a better cutting and engraving software called LightBurn.  The machine currently has provision to control the laser power from the top panel (display with buttons to select the desired power - please see link 'Machine Panel View' below).  Basically the laser power panel generates 0-5V that is sent to the power supply (3-pin input labelled ‘G’ & ‘In’) to control the laser power.  As mentioned the new board works with LightBurn which allows you to control the laser Power through software & it also regulates it when engraving to control contrast and produces much better quality (picture engraving).  I would rather not give LightBurn software max (100%) power because running at lower power will improve the life of the CO2 laser tube.  So if I bypass the machine power panel and connect the new controller to the power supply I will not be able to limit the max power.  One solution that I have come up is to take the two voltages produced by the panel and the controller, compare and then always select the lowest voltage and send that voltage to the laser machine power supply.  This way I can still limit the laser power from the panel even if I select 100% within LightBurn.  I am at a loss on how to design this comparator circuit and need your help please.  when researching for a solution, I saw something that had comparator and two MOSFETs (P-Channel and N-CHannel MOSFETs) but they did not provided a lot of details on what part numbers to use or how the Comparator Supply lines should be connected  (I do have 5V on Laser power supply, that is what I am showing below in my crude circuit).  The Gates are tied together and fed to comparator out.  The Drains are connected together and is the out voltage that will go to laser power supply.  The Sources are tied back to Vin 1 & 2.



                                                                 MOSFET 1
                                                                     |------   S (tied to Vin 1)
                            5V?                  |---------|    |
   Vin 1 -----|..........|.........           |          G     |-------  D
                  |  Comparator  |------- |                          |------------  Drains tied together is V-out
   Vin 2------|..........|..........  out    |                 |------ D     
                              GND?             |---------|    |
                                                               G    |------   S (tied to Vin2)   
                                                                 MOSFET 2

My Request from you
Can someone please help me with:
a) if this solution and circuit is complete (no caps or resistors needed) and will/should it actually  work
b) what comparator and MOSFETs to select please


Links:

OMTech K40 Laser: https://www.amazon.com/gp/product/B01EJDH1BO/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
   Machine Panel View:  https://m.media-amazon.com/images/I/81aZrCXVQ1L.jpg
   Laser Power board and stock Controller: https://ae01.alicdn.com/kf/HTB1V70iJVXXXXbpXXXXq6xXFXXXK/LIHUIYU-Main-Board-M2-Nano-Co2-Laser-Control-System-Engraver-Cutter-K40-Dongle-B-Control-Panel.jpg_640x640.jpg
   Stock Power Supply: https://1.bp.blogspot.com/-vOmLiPap6OI/V18lmsdHb_I/AAAAAAAAZ3s/tBolbK-Ypg0bH-8kJATqTJPHiDn3ST43wCKgB/s1600/20160613_145220.jpg


Manport Controller Board: https://monportlaser.com/collections/accessories/products/grbl-control-board?msclkid=206154b97c7d13f684d630bff6c15f11&utm_source=bing&utm_medium=cpc&utm_campaign=40wLB%2B%E5%A4%A7%E8%AF%8D&utm_term=k40%20control%20board%20upgrade&utm_content=motherboard

Many Thanks,
Masood

[Ian.M]

In theory that approach could somewhat work, but in practice you run into problems with arranging the MOSFETs so the body diodes never conduct, and with their gate drive.  Generally a MOSFET requires approximately double its max. gate threshold voltage to turn on properly, and  the gate-source voltage needs to be under half the min. gate threshold voltage to turn off properly, and I think that with both source voltages varying rail to rail, and the gate drive shared by both MOSFETs you risk neither MOSFET turning on, or possibly even both turning on!

Consider using a SPDT analog switch IC controlled by the comparator.

Alternatively, have you considered that it may be preferable to multiply the two control voltages so Lightburn's 0%-100% becomes proportionately 0%-80% or whatever upper limit you set on the machine control panel?